Electrostatic sizing of materials



April 23, 4 H. B. J OHNSON 2,398,792

ELECTROSTATIC SIZING- OF MATERIALS Filed Oct. 22, 1943 200 a; 100 my 10050 J0 Mes/ Qza ucGDrS .50 22 10 Mes/Z V 1 N1 EN TOR. f/erZarZBJoZuzsonPatented Apr. 2a, 1946 2,398,792

UNITED STATES PATENT OFFICE ELECTROSTATIC SIZING or MATERIALS Herbert B.Johnson, Rochester, N. Y., assignor to Bitter Products Corporation,Rochester, N. Y., a corporation oi New York Application October 22,1943, Serial No. 507,288 6 Claims. (Cl. 209-127) This invention relatesto the dry sizing, gradpurpose of reducing the load on the differenting, or classification of mixed particles of masized screens, so as togreatly increase the lift of terial according to their diiierent sizesand in the screen or bolting cloth and assure more accutheir naturalstate, without separation of the rate sizing of the finished product tomeet very particles according to their electrostatic suscep- 5 closeUnited States or Tyler standard specifitibility based on chemicalcomposition or other cations. difierence in kind, and one object of theinven- Many groups of mixed particles, such as feldtion is to provide animproved and commercially spar, fiuorspar, mica, biotite, nepheline, andthe useful process for eflectlng such sizing, without like, containparticles of various-shapes, such as the use of the screen clothscommonly employed n sliver, flat particles and particles with roughirfor the purpose. regular edges which, in passing over the usual In thescreening of fine powders and other mas re n 010th. tend to Wedge in andquickly blind terials containing particles which accumulate a the r npen n s and r a ly r e h surface static charge when coming into contactr ning icien y. Another object of the p eswith vibrating screen cloth ofvarious materials, is cut invention is to provide an imp m h such asmetals, silk, and the like, the static for sizing materials t in s hparticles so charge so accumulated causes the particles to agas to avoidsuch diillcultles and increase the glomerate, clutter together, or formballs which Screening effieiencycannot pass through the screen openings.Such Another Object is o provide an improved pr difllculties not onlytend to greatly reduce the m ess f l s g partic es acc rding to size. oscreening capacity and greatly increase the cost, future treatment suchas the concentration or but also to lower the screening eiliciency, andSeparation of i d Par i s ac r t th i another object of the presentinvention is toprodifiel'ences in pe ific gravity, magnetic P vide animproved process for obviating these difbiiity. electrostaticsusceptibility. or particular ilculties in the sizing oisuch materials.is Shape- Another object is to provide an improved proc- A further (Meetis Pmvide an impmved and egg for sizing many materials of a highlyhygfl)- less expensive method for removing from many scopic nature whichdue to th rapid accumumaterials, requiring further treatment 01'balllation of moisture, tend to agglomerate, cluster, dung, the finedust Particles, such as would Pass ball up and block the openings ofscreen cloth. 8 a hundred mesh screen, eliminating the Many chemicalsalts, such as common salt, 'potdust nuisance and Wing the Wanting ashsalts and the like, are of a highly corrosive dmmlsin handling suchmaterialsnature which causes corrosion and rust that not Stm furtherobject is to Provide a more only blinds the screen cloth openings, butinjects cient process for removing the finer sized parinto the materialimpurities in the form of rust 5 times Produced during Particlereduction, crush scales which seriously impair the appearance of 111ggrinding ns, t ach step of the rethe screen products and defeat thepurpose of duction, 80 that 8. minimum 8111011111", Of inpalpableproducin chemically pure and marketable proddust or slimes is producedin crushing ores, rocks, ucts. Another object of the invention is toafchemical Salts, and many food Productsford an improved process forsizing such 40 To these and other ends the invention resides givematerials so as t avoid t m mt in certain improvements and combinationsof fen-ed t parts, all as will be hereinafter more fully de- Anotherobject of t invention is t pruvide a scribed, the novel features beingpointed out in the process for removing, t minimum breakage claims atthe end of the specification. handling, the finer sizes 01' crystals ofpotash and 4s Inthe drawing: many other salts which frequently have alower Fig. 1 is an elevational view. p rtly in section value and causethe coarser cyrstals to cake or and P y mm i s w a h d an cementtogether so as to impair the appearance apparatus for separating andclassifying particles of the crystal salts and seriously interfere withof finely divided materials according to size, cmthe free-flowingqualities desired in handling and 50 bodying the present invention, andpackaging operations. Fig. 2 is a similar view showing repeated appli-Another object is to provide an advantageous cation of the invention toeffect a succession of method for classifying particles according tosize separations in diflerent size classifications. for further accuratescreening with standard It has been known heretofore that the particlesmesh screen cloth or silk bolting cloth, for the as c! a mixture offinely divided materials may be separated by delivering the mixture on arotary conveying electrode into a strong electric field produced betweenthe conveying electrode and a second electrode spaced therefrom andhaving a smooth surface and substantial size and charged to a highpotential relative to the conveying electrode of, say, 10,000 volts ormore, so as to cause the particles of better conducting material,substantially irrespective of size, to be repelled from the conveyingelectrode and thereby separated from particles of material of lowerconductivity, as described, for example, in my prior United StatesPatents Nos. 2,135,716 and 2,246,253. In another known type of methodand apparatus, the electrode spaced from the conveying electrode hasbeen of small size, such as fine wire, or provided with sharp points,and charged to a similarly high potential relative to the conveyingelectrode so as to produce a spray discharge and ion bombardment of theparticles on the conveying electrode, with the result that the particlesof materials of lower conductivity, substantially irrespective of size,are caused to cling to the conveying electrode so as to be depressed andseparated from the better conducting material.

The present invention employs neither of the above types of method andapparatus, involving instead a relatively weak electric field on theconveying electrode produced by a spaced electrode of intermediate sizeand having a relatively low potential difference from the conveyingelectrode, so as to avoid differential action on particles of differentconductivity but effecting a moderate depressing action on the particlesof smaller size, as the particlesof larger size are moved in theopposite direction by the centrifugal force imparted by the conveyingelectrode, thereby advantageously coordinating and combining theopposite effects of a suitable electric field and centrifugal force toclassify the particles according to size.

I have discovered, more particularly that the particles of a finelydivided mixture may be separated and classified according to the sizesof the particles, irrespective of their materials and resultingelectrostatic susceptibility, by a combination and coordination ofcentrifugal force and electrostatic force, through control andcoordination of the peripheral speed of the rotary conveying electrodeand of the potential difference between it and the charged electrode, inconJunction with electrode surfaces of suitable characteristics.

The centrifugal force applied by a rotary electrode tends to impel theparticles of greater mass forwardly to the outside of the particlestream falling from the electrode, while the particles of less mass tendto drop more nearly vertically at the rear side of the stream, with theresult of a partial separation and distribution of the particlesaccording to their relative mass and size. Such centrifugal separation,however, has been found insufilcient and inadequate to assure acontinuous and even spread of the particles according to size, and Ihave found that such centrifugal sizing action may be supplemented andrendered successful for the purpose by applying against it a coordinatedelectrostatic force of suitable character. Investigations involving manytests on metallic and non-metallic minerals, chemical salts, and othermaterials, have proven that the surfaces and diameters of the electrodesand their difference in potential are important factors in applying theelectrostatic force against the centrifugal force, in order to obtainemcient sizing action without producing any separation of the particlesaccording to their different kinds of materials and electrostaticsusceptibility.

In the application of electrostatic forces as used in the electrostaticseparation of particles according to differences in the materials ofwhich they are composed, it has been found that the separating action isreversed as the diameter of the charged electrode is increased from .005of an inch to 2 or 3 inches. For example, if a fine wire having adiameter up to 6 inch is used as the charged electrode, the resultingion bombardment causes the particles to stick to the surface of thegrounded electrode. As. the diameter of the charged electrode increasesfrom to 2 inches, or more, there is an attracting action of the chargedelectrode and a repelling'of the more conductive particles from thegrounded electrode.

It has been found through many experiments and tests that chargedelectrodes ranging in diameter from inch to of an inch have a moreneutral repelling or attracting action and that there is a definiterelationship between the charged electrode diameters ranging from 1 6inch to inch and the potential difference employed between the chargedand grounded electrodes. For example, ,in employing charged electrodesranging in diameter from 1*; inch to inch, there is practically noelectrostatic separation of the particles according to the material ofwhich they are composed, or interference with the free sizing action ofthe particles, if the potential is maintained below about 7,000 volts.If the potential difference is increased above about 7,000 volts,electrostatic separation of the particles according to their materialsbegins to take place and increases in proportion tothe increase in thedifference in potential between the electrodes, so as to defeat thedesired separation according to size.

My improved method is best described in connection with a suitableapparatus for carrying it out, shown in Fig. 1 of the drawing, in which35 is a feed hopper for the finely divided mixture of particles to besized. At 36 is an adjustable fee'd gate, of any known or suitablevariety, for controlling the rate of delivery of the particles to thesurface of a rotary cylindrical electrode 31 of substantial size havinga diameter of several inches. The surface of electrode 31 is preferablyknurled, fluted, corrugated or otherwise roughened, with suitablereference to the size of particles to be treated and the type ofmaterials to be handled. Electrode 31 is rotated at a suitableperipheral speed for picking up, rotating and discharging the particlesat the same speed into an electrical field produced between electrode31, which is grounded as at 38, and an electrode 39 connected as at 40with a suitable potential source such as a rectifier unit of the usualor any suitable variety, energized by a transformer connected through areversible switch with a service power line, as conveniently-indicatedin connection with Fig. 2. The difference in voltage between electrodes31 and 39 is of lower range'than that commonly employed in electrostaticseparation, ranging to not more than about 7,000 volts.

Electrode 39 preferably has a smooth cylindrical surface and a diameterof not less than 1 g of an inch and not more than about of an inch, andmay be either rotating or stationary. It is located, as shown, insuitably spaced relation with the point in which the particles drop fromthe conveying electrode 31, preferably with the centers of the twoelectrodes in a common horizontal plane.

In carrying out the present process by the described apparatus shown inFig. 1, the rotary electrode 3'! is given a peripheral speed adapted todeliver the particles in a definite stream with the particles thereofsubjected to centrifugal forces proportional to their respective massesand tending to project them somewhat forwardly from the point ofdelivery from the electrode. The electrostatic force imposed byelectrode 39 operates in opposition to such centrifugal force, ininverse ratio to the sizes of the particles, so that they are depressedbackwardly and fanned out, as shown, with the finer particles fallingfarthest to the rear, as at 42. A series of dividers 43 are preferablyprovided and adjusted to separately collect the particles in eachparticular size range, as shown. Any known or suitable means may beprovided, such as the variable resistance indicated at 64, for varyingthe voltage applied to electrode 39.

The peripheral speed of the conveying electrode 37 and the particularvoltage of the charged electrode 39 are selectively adjusted andcoordinated with each other for operation at best efiiciency to suitvariations in the particle size and particular kind of material to behandled. The roughened surface of the conveying electrode operateseffectively to discharge the particles at a corresponding surface speed,without any substantial tendency to set up spray discharge or, ionbombardment, because of the relatively low voltage of the chargedelectrode 39. The charged electrode is of such impressed voltage andsize asto produce a limited depression of the particles in inverse ratioto their size and in opposition to the applied centrifugal force, sothat the particlesare separated'and fanned out according to size withoutany tendency to separation according to the kind of material and itselectrostatic susceptibility.

I have found, for example, that with a roughened, grounded, conveyingelectrode of several inches in diameter, rotating at a peripheral speedbetween, say, 100 to 450 feet per minute, and a charged electrode a; ofan inch in diameter at a potential of 3500 volts, four sizeclassifications are easily produced when treating phosphate rock,phosphate flotation concentrates, copper stamp mill tailings, cementrock, potash salts, feldspar, fiuorspar, abrasives and other metallicand nonmetallic ores and rocks, which have been preliminarily crushed topass a six mesh screen and surface dried. With the voltage and otherdetails of the exemplary embodiment here referred to, the chargedelectrode 39 is preferably located with its center spaced about an inchfrom the material on the conveying electrode, but different materialsand different preliminary particle sizes may require some variation inthe surface roughening and peripheral speed of the conveying electrode,the spacing of the electrodes, the potential of the charged electrodeand perhaps its diameter, but such variations are readily made insetting up the apparatus and calibrating it to the particular operatingconditions, for optimum efficiency, as will be readily apparent to thoseskilled in the art. It may also prove desirable, in connection with somematerials, to reverse the polarity of the charged electrode, asdescribed in my prior Patent No. 2,197,864.

The invention may be applied to repeated treatments of a given material,using a plurality of pairs of electrodes and dividers, as shown in Fig.2, where the feed hopper is shown at 45,

with an adjustable feed gate 44 for feeding a uniform stream of materialto the rotary grounded conveying electrode 41. Opposite electrode 41 isa charged electrode 48, these electrodes having the same construction,arrangement and operation as described in connection with Fig. 1.comprising suitable means for varying the surface speed of the conveyingelectrode and the voltage of the charged electrode. The finest grade ofparticles separated at these electrodes, passing a two hundred meshscreen, for example, is separated by a suitable divider 49, while theremaining rades are received by suitable collecting surfaces indicatedat 50, and fed to a second pair of electrodes BI and 52, similarlyconstructed and operated. The finest grade of particles separated bythis pair of electrodes is separately collected by a divider 53, whilethe remaining grades are received by suitable collecting surfacesindicated generally at 54 and fed to a final pair of electrodes 55 and56. Electrodes 55 and 56 are similarly constructed and operated andproduce a final separation into two particle sizes, indicated at 51 and58, which are separately collected by suitable means comprising adivider 59, as shown.

It has been demonstrated in practical operation of the invention thatcoordinating adjustments of the centrifugal and electrostatic forcesemployed, in connection with suitable location of the divider, producedefinite sizes of materials which, tested by the usual screen test,indicate at 88-92% sizing efficiency, according to standard screenoperating measurements.

The apparatus employed to carry out the present process may be totallyenclosed, so that the impalpable dust can be removed pneumatically andcollected in the usual way, thereby preventing any accumulation of dustto the point of explosive hazard. The different conveyor speeds and thedifierent polarities and potentials of the electrodes required may bereadily calibrated for various materials and particle sizes for adaptinthe machine to different uses, the controlling factors being well withinthe range of commercial application.

I claim:

1. The process of separating particles according to their differentsizes from a finely divided mixture thereof comprising the steps ofapplying to said mixture a centrifugal force adapted to deliver the samein a definite stream with the particles thereof subjected to said forcein proportion to their respective masses, subjecting said particles inspace to an electrostatic field between cylindrical electrodes having apotential difference of about thirty-five hundred volts acting todepress particles of smaller size without separation of the particlesaccording toconductivity, the smaller of said electrodes having adiameter of not less than approximately one-sixteenth of an inch and notmore than approximately threefourths of an inch, to separate the smallerfrom the larger particles in said stream in opposition to saidcentrifugal force, and separately collecting particles of difi'erentsizes.

, 2. The process of separating particles accordin to their differentsizes from afinely divided mixture thereof comprising the stepsofapplying to said mixture a centrifugal force'adapted to deliver thesame in a definite stream with the particles thereof subjected to saidforce in proportion to their respective masses. subjecting saidparticles in space to an electrostatic field between cylindricalelectrodeshaving a potential the smaller of said electrodes having adiameter of not'less than approximately one-sixteenth of an inch and notmore than approximately threefourths of an inch, to separate the smallerfrom the larger particles in said stream in opposition to saidcentrifugal force, and separately collect- 'ing the particles ofdifferent sizes.

3. The process of separating particles accord-v ing to their differentsizes from a finely divided mixture thereof comprising the steps offeeding said mixture on a cylindrical electrode rotating at a speedadapted to deliver the same in a definite stream with the particlesthereof subjected to centrifugal forces in proportion to their 'respec-,tive masses, delivering said stream into an electrostatic field formedbetween said electrode and a smaller cylindrical electrode having apotential difference therefrom of not more than about 7,000 volts, saidsmaller electrode having a diameter of not less than about one-sixteenthof an inch and not more than about three-fourths of an inch, to separatethe smaller from the larger particles in said stream in opposition tosaid centrifugalforce, and separately collecting the particles ofdifferent sizes.

4. The process of separating particles accord- I ing to their differentsizes from a finely divided mixture thereof comprising the steps of.conveying said mixture on a cylindrical electrode having a roughenedsurface and rotated at a speed adapted to deliver the same in a definiteinch, and separately collecting theparticlesof different sizes.

7 diflerence of not more than substantially 7,000 volts,

5. The process of separating particles according to their differentsizes from a finely divided mixture thereof comprising the stepsoffeeding said mixture on a cylindrical electrode rotating at a speedadapted to'deliver the same in a definite stream with the particlesthereof subjected to centrifugal forces in proportion to theirrespective masses, delivering said stream into an electrostatic fieldformed between said electrode and a smaller cylindrical electrode havinga potential difference therefrom of not more than about seven thousandvolts acting to depress particles of smaller size without separation ofthe particles according to conductivity, said smaller electrode having adiameter of not less than about onesixteenth of an inch and not morethan about three-fourths of an inch and said electrodes having theircenters substantially in the same horizontal plane, to separate thesmaller from the larger particles insaid' stream in opposition to saidcentrifugal. force and separately collecting the particles of differentsizes.

6. The process of separating particles according totheir different sizesfrom a finely divided mixture thereof comprising the steps of conveyingsaid mixture on a cyilndrical electrode having a roughened surface androtated at a speed adapted to deliver the same in a definite stream withthe particles thereofsubjected to centrifugal forces proportional totheir respective masses, delivering said stream into an electrostaticfield formed between said electrode and a smaller cylindrical electrodehaving its center located substantially inthe same horizontal plane withthe center of said conveying electrodeand located about one inch fromthe surface of said conveying electrode, maintaining a potentialdifference between said electrodes of not more than approximately 7000volts, said smaller electrode having a smooth surface and a diameter ofnot less than approximately one-sixteenth of an inch and not more thanapproximately three-fourths of an inch, and separately collecting theparticles of different sizes.

HERBERT B. JOHNSON.

